1. Field of the Invention
The present invention relates to a wireless communication system for mutual communication among a plurality of wireless stations such as a wireless LAN (Local Area Network), a wireless communication apparatus, a wireless communication method and a computer program, and more particularly to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which a wireless network is configured by each station operating in a self-organized distributed manner.
More particularly, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program in which communication stations carry out data communication by access control based on CSMA (Carrier Sense Multiple Access) in an environment of self-organized distribution type network, and in particular to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program which realizes short packet broadcast communication with a short latency.
2. Description of Related Art
As one of the standard specifications of wireless networks, IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., refer to Non-patent Document 1), HiperLAN/2 (e.g., refer to Non-patent Document 2 or Non-patent Document 3), IEEE 802.15.3, Bluetooth communication and the like can be enumerated. The IEEE 802.11 standard has various wireless communication schemes such as the IEEE 802.11a standard and the IEEE 802.11b standard depending upon a difference of a wireless communication scheme and a frequency band in use.
In general, in order to configure a local area network by using wireless technologies, a method is used by which one apparatus to be used as a control station called an “access point” or a “coordinator” is installed in an area and a network is formed under the collective control by the control station.
In a wireless network having distributed access points, in a case where information is transmitted from a certain communication apparatus, an access control method based on bandwidth reservation has been adopted widely by which a band necessary for transmitting the information is first reserved at an access point to use a transmission path without collision of information transmission with other communication apparatuses. Namely, synchronous wireless communication is performed by mutually synchronizing with communication apparatuses in the wireless network by distributing access points.
In a case where asynchronous communication is to be performed between communication apparatuses on the transmission side and reception side in a wireless communication system having access points, this wireless communication requires by all means wireless communication via an access point so that there arises the problem that a transmission path use efficiency is decreased by half.
As another method of configuring a wireless network, “Ad-hoc communication” has been devised in which terminals perform wireless communication directly and asynchronously. It can be considered that the ad hoc communication in which arbitrary terminals perform wireless communication directly without using a particular access point is suitable particularly for a small scale wireless network configured with a relatively small number of clients positioned near each other.
For example, in a wireless LAN system of IEEE802.11 system, there is prepared an ad hoc mode which operates peer to peer in a self-organized manner without having a relation of a controlling station and a controlled station. In this operation mode, when a beacon transmission time comes, each terminal starts counting a random time period and in a case where it receives no beacon from another terminal by the end of the time period, it transmits a beacon.
Now, IEEE 802.11 is exemplified to describe the details of the conventional wireless networking.
The networking in the IEEE 802.11 is based on the concept of a basic service set (BSS). The BSS is composed of two kinds of modes, namely BSS defined by an infrastructure mode, in which a master such as an access point (AP) functioning as a controlling station exists, and an ad hoc mode composed of only a plurality of mobile terminals (MTs) functioning as mobile stations.
<Infrastructure Mode>
Referring to FIG. 26, the operation of the IEEE 802.11 at the time of the infrastructure mode is described. In the BSS at the time of the infrastructure mode, an AP performing coordination is indispensable in a wireless communication system.
The AP arranges a range in which radio waves can reach around a peripheral region of a local station as BSS, and configures a “cell” so referred to in a so-called cellular system. An MT existing in the neighbor of the AP is contained by the AP to enter the network as a member of the BSS. That is to say, the AP transmits a control signal called as a beacon at an appropriate time interval, and an MT capable of receiving the beacon recognizes the existence of the AP in its vicinity, and further the MT performs the establishment of a connection with the AP.
In the example shown in FIG. 26, a communication station STA0 operates as an AP, and the other communication stations STA1 and STA2 severally operate as an MT. Hereupon, the communication station STA0 as the AP transmits a beacon at a predetermined time interval as shown in a chart on the right side of the figure. The next transmission time of a beacon is informed in a beacon in a parameter format called as a target beacon transmission time (TBTT). Then, when time comes to the TBTT, the AP operates a beacon transmission procedure.
On the other hand, because the MT can recognize the next beacon transmission time by receiving a beacon and by decoding the TBTT field in the beacon, an MT existing around the AP sometimes enters its sleep state by turning off the power sources of its receiver until the next TBTT or a plurality of times later TBTT (in a case where no necessity exists for receiving).
In a case of the infrastructure mode, only the AP transmits a beacon at a predetermined frame period. On the other hand, the peripheral MT enters the network by receiving the beacons from the AP, and does not transmit any beacons. It is noted that the present invention principally aims to operate a network without intervening by any master controlling station such as the AP, and does not relate to the infrastructure mode directly. Accordingly, the infrastructure mode is not described any more.
<Ad Hoc Mode>
Referring to FIG. 27, the operation of the IEEE 802.11 at the time of the ad hoc mode on the other hand is described.
In an IBSS of the ad hoc mode, an MT defines an IBSS in a self-organized distributed manner after performing a negotiation among a plurality of MT's. When the IBSS has been defined, the MT group determines TBTTs at every fixed interval after negotiations. When each MT recognizes the arrival of a TBTT by referring to a clock in a local station, the MT transmits a beacon in a case where the MT recognizes that no MT has transmitted a beacon yet after a delay of a random time.
In the example shown in FIG. 27, a situation in which two MTs constitute an IBSS is shown. In this case, any one of the MTs belonging to the IBSS transmits a beacon every arrival of a TBTT. Moreover, there is also a case where beacons transmitted from each MT collide with each other.
Moreover, also in the IBSS, the MT's sometimes enter their sleep states in which the power sources of their transmitter-receivers are turned off as occasion demands (which will be described later).
<Transmission-Reception Procedures in IEEE 802.11>
Successively, the transmission and reception procedures in the IEEE 802.11 are described.
In a wireless LAN network in an ad hoc environment, it is generally known that a hidden terminal problem is generated. The hidden terminal means a communication station that, in a case of performing communication between certain specific communication stations, one of the communication stations of the communication party can listen but the other communication station of the communication stations cannot listen. Because the hidden terminals cannot perform any negotiation among them, there is the possibility that transmission operation collides with each other.
As a methodology for solving the hidden terminal problem, CSMA/CA (Carrier Sense Multiple Access Collision Avoidance) in accordance with a RTS/CTS (Request To Send/Clear To Send) procedure is known. The IEEE 802.11 also adopts the methodology.
Hereupon, the CSMA means a connection system for performing a multiple access on the basis of carrier detection. Because it is difficult to receive a signal the information of which has been transmitted by a local station in wireless communication, the confirmation of the nonexistence of the information transmission by other communication apparatuses is performed not in accordance with the CSMA/collision detection (CD) but in accordance with CSMA/collision avoidance (CA) system, and then its own information transmission is started. Thereby collisions can be avoided.
Moreover, in the RTS/CTS system, a communication station of a data transmission source transmits a transmission request packet RTS (request to send), and starts to transmit data in response to the reception of a confirmation information packet CTS (clear to send) from a communication station of a data transmission destination. Then, when the hidden terminal receives at least one of the RTS or the CTS, the hidden terminal can avoid a collision by setting a transmission suspension period of the local station for a period in which data transmission based on the RTS/CTS procedure is expected to be performed.
FIG. 28 shows an operation example of the RTS/CTS procedure. Incidentally, in the wireless communication environment, communication apparatus #1, #2, #3 and #4 are arranged to be in a state in which the communication apparatus #1 can communicate with its neighbor communication apparatus #2, and in which the communication apparatus #2 can communicate with its neighbor communication apparatus #1 and #3, and further in which the communication apparatus #3 can communicate with its neighbor communication apparatus #2 and #4, and still further in which the communication apparatus #4 can communicate with the neighbor communication apparatus #3, but in which the communication apparatus #1 is a hidden terminal for the communication apparatus #3, and further in which the communication apparatus #4 is a hidden terminal for the communication apparatus #2.
In the example shown in the figure, the communication apparatus #2, which transmits data, transmits a transmission request (RTS) to the communication apparatus #3, and the communication apparatus #3 sends back confirmation information (CTS) to the communication apparatus #2.
At this time, the communication apparatus #1 and #4, which are in the situation in which they can be hidden terminals from each of the communication apparatus #2 and #3, detect the use of transmission paths and perform the control of not performing transmission until the completion of the communication. In specific, the communication apparatus #1 detects the start of the data transmission of the communication apparatus #2 of a transmission source on the basis of an RTS packet, and thereby the communication apparatus #1 can recognize that the transmission path has been used already for a period from that to the completion of the successive transmission of the data packet. Moreover, the communication apparatus #4 detects the start of a data transmission the reception destination of which is the communication apparatus #3 on the basis of a CTS packet, and thereby the communication apparatus #4 can recognize that the transmission path has been already used for a period until the communication apparatus #4 detects the sending back of an ACK packet from the communication apparatus #3.
It is noted that, in a case where another communication apparatus happens to transmit some signal almost at the same time when the communication apparatus #2 of an information transmission source transmits the RTS, the signals collide with each other. Consequently, the communication apparatus #3 of the information reception destination cannot receive the RTS. In this case, the communication apparatus #3 does not send back the CTS. As a result, the communication apparatus #2 can recognize that the RTS has collided because the communication apparatus #2 does not receive the CTS for a while. Then, the communication apparatus #2 starts the procedure of the retransmission of the RTS while making random backoff effective. Basically, the communication apparatuses contend each other to acquire a transmission right while having a risk of a collision as above.
<Access Competing Method in IEEE 802.11>
Successively, an access competing method prescribed in the IEEE 802.11 is described.
In the IEEE 802.11, four kinds of packet intervals (IFS: inter frame space) are defined. Hereupon, three IFSs of them is described with reference to FIG. 29. As the IFSs, SIFS (short IFS), PIFS (PCF IFS) and DIFS (DCF IFS) are defined in the order of shortness.
In the IEEE 802.11, as a basic medium access procedure, the CSMA is adopted (as described above). Before a transmitter transmits something, the transmitter operates a timer of the backoff for a random time while monitoring a medium state, and the transmission right is not given to the transmitter until the state of the nonexistence of transmission signals during that period is confirmed.
When an ordinary packet is transmitted in accordance with the CSMA procedure (called as a distributed coordination function (DCF)), the medium state is first monitored only for DIFS after the transmission of some packet has been completed. In a case where no transmission signals exist during the period, the random backoff is performed. Moreover, in a case where no transmission signals exist also during the period of the random backoff, the transmission right is given to the transmitter.
On the other hand, when a packet having exceptionally high urgency such as an ACK is transmitted, the packet is allowed to be transmitted after the SIFS packet interval. Consequently, a packet having high urgency can be transmitted before a packet to be transmitted in accordance of the ordinary CSMA procedure.
In short, the reason why different kinds of packet interval IFS's are defined is that the priority setting of the transmission right competition of packets is performed according to which one of the SIFS, the PIFS and the DIFS the IFS is, namely according to the length of the packet interval.
<Signal Transmission/Reception Procedures in Sleep State>
In the networking in the IEEE 802.11, an MT sometimes enters a sleep state, in which the MT turns off the power source of the transmitter/receiver thereof even in a case of the IBSS at the time of the ad hoc mode as occasion demands. The processing procedure in this case is described with reference to FIG. 30.
In a case where the sleep mode is applied in the IBSS in the IEEE 802.11, a time zone for a while from a TBTT is defined as an announcement traffic indication message (ATIM) window. In the ATIM window time zone, all of the MTs belonging to the IBSS have their receivers operating, and consequently as long as in this time zone, even an MT operating in the sleep mode thereof can basically perform reception.
In the case where the local station includes the information addressed to someone, each MT notifies a reception side of that the local station is holding the transmission information, by transmitting an ATIM packet to a communication party in the ATIM window time zone. An MT which has received the ATIM packet has the receiver thereof operating until the completion of the reception from the station which has transmitted the ATIM packet.
In the example shown in FIG. 30, a case where three MTs STA1, STA2 and STA3 exist in the IBSS is exemplified. When a TBTT arrives, each of the MTs STA1, STA2 and STA3 monitors a medium state for a random time while having the timer of the backoff operating. In the example shown in the figure, the backoff timer of the MT STA1 terminates at the earliest time, and then the MT STA1 transmits a beacon. Because the MT STA1 has transmitted the beacon, the MTs STA2 and STA3, which have received the beacon, do not transmit any beacon.
Moreover, in the example shown in FIG. 30, the MT STA1 holds the information addressed to the MT STA2, and the MT STA2 holds the information addressed to the MT STA3. In this case, after the MTs STA1 and STA2 have transmitted/received beacons, the MTs STA1 and STA2 again severally monitor the medium state for a random time while having their timers of backoff operating. In the example shown in the figure, because the timer of the MT STA2 has terminated earlier, an ATIM message is first transmitted from the MT STA2 to the MT STA3. When the MT STA3 receives the ATIM message, the MT STA3 feeds back the fact of the reception to the MT STA2 by transmitting an acknowledgment (ACK) packet to the MT STA2.
After the completion of the transmission of the ACK packet from the MT STA3, the MT STA1 further monitors each medium state for a random time while having the timer of the backoff operating. When the backoff timer terminates, the MT STA1 transmits an ATIM packet to the MT STA2. The MT STA2 performs feedback to the MT STA1 by sending back an ACK packet indicating the reception of the ATIM packet.
After such exchanges of the ATIM packet and the ACK packet have been performed in the ATIM window, even in the following sections, the STA3 has the receiver thereof operating for receiving the information from the MT STA2, and similarly the MT STA2 has the receiver thereof operating for receiving the information from the MT STA1.
The MTs STA1 and STA2, both holding transmission information, severally monitor the medium state for a random time when the ATIM window has ended while having the timer of the backoff operating. In an example shown in FIG. 30, because the timer of the MT STA2 has terminated earlier, the information addressed to the MT STA3 has been transmitted from the MT STA2 to the MT STA3 earlier. After the completion of the transmission, the MT STA1 again monitors the medium state for a random time while having the timer of the backoff operating. When the timer terminates, the MT STA1 transmits a packet to the MT STA2.
In the procedure described above, a communication station which does not receive any ATIM packets in the ATIM window or a communication station which holds no information addressed to anyone can turn off the power source of the transmitter/receiver thereof until the next TBTT, and thereby can reduce the power consumption thereof.
[Non-Patent Document 1]
International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification
[Non-Patent Document 2]
ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-Patent Document 3]
ETSI Standard ETSI TS 101-761-2 V1.3.1 Broadband Radio Access Network (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
(1) Congeniality with Broadcast Communication
In the IEEE 802.11, communication addressed to Broadcast/Multicast is defined. In this case, however, a reply of an ACK packet being a reception confirmation response signal is not performed. Because the basic access method is the CSMA, there is a possibility that transmission signals collide with each other, but there is no means for judging the signal collision in the broadcast communication of the ad hoc mode. Moreover, it is also considerable to transmit directional data to a plurality of stations by a plurality of times. In this case, because the data is transmitted to specific stations, reception confirmation responses can be obtained. However, the method of transmitting the same data many times always with risks of the collision with the transmission signals of the other stations is not efficient. As described above, the broadcast communication in the ad hoc mode generally has not good congeniality.
(2) Congeniality with Short Packet
In the IEEE 802.11, a fixed overhead is generated at a head of a packet irrespective of the data quantity to be transmitted. In a case where the data quantity to be transmitted by one packet is large, the ratio of the overhead against the packet quantity is small. However, when the data quantity to be transmitted is small, a phenomenon in which the overhead is larger than a data area is produced according to circumstances, and then resource use efficiency is remarkably lowered.
(3) Congeniality with Traffic Requiring Short Latency
As described above, the IEEE 802.11 is a system defined on the premise of allowing the generation of data collisions by the CSMA, and the data collision is covered by retransmission. At the time of performing the retransmission, the random backoff is again set, and the backoff value is set to become larger by the rate of power of two every retransmission. Consequently, in the environment in which a plurality of communication stations tries to perform transmission at the same time, there is the possibility that some delays are produced until the data transmission of the communication stations is completed. Consequently, a problem arises at the time of traffic accommodation requiring a short latency.
(4) Lowering Power Consumption
In the IEEE 802.11, a power saving mode as a low power consumption use is defined. A station which has transmitted a beacon needs to have the receiver thereof operating until the ATIM window which is started continuously to the next beacon transmission ends. For example, in a case where an IBSS is configured between two stations, it is necessary to have their receivers operating at an hour rate of 50% or more statistically irrespective of the existence of data transmission. Consequently, the effect of lowering the power consumption is low. For example, in an application applying wireless communication to game controllers in a case where a plurality of users play a match, it is necessary to perform the broadcast delivery of a command input into one machine to the controllers of all of the users. Hereupon, individual input command is formed as a short packet, but a short latency is required for proceeding the processing of a game. Moreover, it is expected that the game controllers are severally driven by a battery, and then the lowering of the power consumption of the game controllers is desired. Accordingly, the game controllers are in their sleep states except for the time of packet transmission and reception.